1. Field of the Invention
This invention relates to a process of manufacturing rolls of continuous synthetic thermoplastic film consisting of extruded flat film or of extruded, blow-formed and flattened tubular film wherein the thickness of the film is measured over the width of the film and any longitudinal striplike portions which deviate in thickness from a given thickness of the film are adjusted to restore the deviant striplike portion to said mean thickness by adjusting the temperatures of juxtaposed temperature control sectors of the die slot corresponding to the striplike portions of the film or by changing or varying the width or opening of corresponding portions or segments of the extrusion slot or mouth of the extruding die.
2. Description of the Prior Art
In the manufacture of extruded, blow-formed and flattened tubular films and of extruded flat films, it is not possible to avoid a deviation of the thickness of the continuous film from the mean thickness of the film over the width of the film. When such films are wound up to form rolls of film, the resulting rolls will be formed with annular bulges as a result of the superposition of relatively thick and/or relatively thin portions. Rolls of film having such annular bulges have an unpleasing appearance and owing to the bulges the continuous film when unwound from the roll will not lie perfectly flat on a support so that difficulties may arise during the subsequent processing.
German patent specification No. 20 35 584 discloses an apparatus and a process for making cylindrical rolls from extruded, blow-formed and flattened tubular film. In that apparatus and process, inflated blown tubular film is flattened by means of flattening plates, which are reversingly moved through 360.degree. so that any relatively thick and relatively thin portions of the tubular film will be distributed in the manner in which a rope is wound when the flattened tubular film is being wound to form a roll.
In the process known from German patent specification No. 20 35 584, relatively thick and/or relatively thin portions of the film are merely distributed over the width of the roll of film so that cylindrical rolls are obtained. But such relatively thick and/or relatively thin portions of the film are not eliminated. On the other hand, processes are known, e.g., from Published German application No. 29 47 293 and U.S. Pat. No. 4,426,239, which disclose how relatively thick and relatively thin longitudinal portions of the continuous films are eliminated by a particular automatic control with reference to the mean thickness of the film in order to achieve rolls which are formed as perfectly cylindrical as possible. However, practice of these processes can still result in the flat film or flattened tubular film comprising a longitudinally extending strip-shaped portion which is relatively thick and, therefore, such excessive thickness can build up during the superposition of the wound convolutions to form an annular bulge in the roll. Even though a relatively thick portion may be eliminated by an automatic control with reference to the mean thickness of the film, any previously formed annular bulge will still deform the convolutions of subsequently wound film adjusted to the mean film thickness and, hence, those convolutions will thus be formed with bulges, which when the film is unwound will be preserved as cambered portions, giving rise to difficulties in the processing of the film.